my understanding (from another thread here) is that A03 or above bios ug permits booting off of sd card. i haven't been able to confirm that yet personally, but that would be the first thing to look into as i see it.
the other thing is some folks have had problems with sdhc cards working with osx. supposedly this is supported, but i've seen some comments here regarding that.
you might also bear in mind that transfer rates on sd are quite a bit slower than usb 2.0. you can look at the charts on wikipedia if you want to know more. this may not be a concern for you, but were it me this would be an unacceptable trade off.